1) Field of the Invention
The invention relates to the use of aqueous polymer dispersions based on acrylate-methacrylate copolymers as lamination adhesives for glossy films.
2) Background Art
The lamination of printed papers or printed board with plastic films for the purpose of print finishing is a process which has been practiced for some time. Printed products are in this way protected against mechanical influences, are given a more attractive appearance, and can be presented in a visually more appealing manner due to the surface gloss thus achieved. Practical examples thereof are record sleeves, book covers, paperbacks, packaging materials for cosmetic articles and advertising materials.
The process of coating and bonding printed paper products by means of plastic films is known as lamination with glossy films. The adhesive employed is accordingly known as a lamination adhesive for glossy films and has the task of bonding the plastic film on the one hand to the printed paper on the other hand. At the beginning of this technological development, exclusively solvent-containing polymer resins were used, which were used either in one-component polyacrylate-based form or in two-component form as reactive polyurethanes. More stringent environmental protection regulations increasingly forced the avoidance of these solvent-containing systems and favored the use of aqueous polymer dispersions. In the case of the lamination films used, the trend increasingly developed in favor of polyolefin films, for example polyfilms (PE films) or oriented polypropylene films (OPP films). These are either flame- or corona-pretreated on their surface in order to improve the adhesion.
The first proposals for bonding flame- or corona-pretreated OPP films to printed paper or printed cardboard packaging by means of aqueous polymer dispersions are described in JP-A 77/42532 (Derwent Abstract) and in EP-A 46823 (U.S. Pat. No. 4,377,433). Both publications essentially describe the use of aqueous vinyl acetate-ethylene copolymer dispersions in which epoxide and amine functions have been incorporated as reactive components. At the time of use, the epoxide-containing and amine-containing vinyl acetate/ethylene dispersions are mixed with one another. These systems have a relatively short pot life and must be used immediately after mixing (2-component adhesive).
All other proposals for improving the bond strengths between polyolefin films and printed paper or board are also characterized by the use of reactive crosslinking agent systems. For example, in JP-A 60/112874, ethylene-vinyl acetate-acrylate dispersions are provided with good adhesion properties with the aid of water-soluble polyamine compounds. DE-A 3642485 (U.S. Pat. No. 4,980,404) recommends acrylate dispersions containing polyamino compounds for bonding plastic films to paper or board. DE-4117487 uses carbonylcontaining polymer dispersions which are crosslinked with the aid of multifunctional aminooxy compounds and thus achieve a significant improvement in adhesion on the corona-treated polyolefin surfaces. Other reactive systems which have been proposed are lattices containing itaconic acid (EP-A 307050). Water-soluble polymers have also been proposed for improving the adhesion in the lamination of glossy films in DD-A 254949 and U.S. Pat. No. 3,974,112.
WO-A 92/12213 describes the use of aqueous dispersions of copolymers of (meth)acrylates of .alpha..beta.-unsaturated carboxylic acids and (meth)acrylates of alkylsulfonic acids as adhesives for the lamination of substrates, where the substrates to be bonded should preferably be selected from the group consisting of polymer films, printed paper, metal-coated paper and metal foils. The systems described therein were intended to replace solvent-containing adhesives or hot-melt adhesives in the production of laminates. The specific problems of lamination with glossy films (gloss and transparency of the laminates) are not discussed.
The object of the invention was to develop an aqueous polymer dispersion which guarantees good bond strength between an optionally corona- or flame-pretreated polyolefin film on the one hand and printed paper or printed cardboard packaging on the other hand, even without the above-mentioned reactive crosslinking agent systems, to give laminates having excellent gloss and transparency.